So-called "green" or freshly-cut lumber must be seasoned by drying the lumber to reduce the moisture content, before the lumber can be put to use. Although lumber can be dried by exposure to ambient air, that practice usually takes months to accomplish and the results seldom are uniform from one batch of lumber to the next. For these reasons, lumber is usually dried in a kiln where the temperature and relative humidity of the air are regulated in an effort to produce the desired moisture content within the lumber in the shortest possible time. Batches of lumber are stacked within the kiln so as to permit significant air circulation through the stack, and one or more such stacks usually are dried within the kiln at a time.
The charge of lumber in a kiln usually is a mix of dense- and coarse-grain lumber, with the relatively dense lumber containing more moisture and therefore requiring more energy (or greater drying time) to reach the desired moisture content. If the entire charge or batch of lumber is dried to meet the moisture-removal requirements of the dense lumber alone, the coarse-grain boards are ruined by overdrying. On the other hand, drying the entire batch of lumber only to reach the proper moisture content for the coarse-grain boards leaves the relatively dense boards with excess moisture, rendering them unsuitable for use without further drying. Although overdrying can be prevented by maintaining atmospheric conditions within the kiln to dry at a relatively slow rate which eventually produces the desired moisture content in the entire batch of lumber, that technique is unacceptably slow for effective commercial use.
Prior art kilns are known which attempt to deal in various ways with the problems of drying mixed-density batches of lumber. These efforts typically involve attempting to maintain a predetermined drying ability within the kiln by maintaining, for example, a constant wet-bulb temperature depression in the air within the kiln. U.S. patents to Reynolds (U.S. Pat. No. 3,386,183) and to Rosenau (U.S. Pat. No. 4,356,641) seek to control the wet-bulb temperature depression by controlling the heat input to the kiln. Gelineau (U.S. Pat. No. 4,599,808) attempts to maintain a predetermined constant rate of evaporation within a kiln by controlling the dry-bulb temperature drop in air flowing across the lumber, while maintaining a constant wetbulb temperature of the air upstream of the lumber. These efforts of the prior art have proven less than fully satisfactory, resulting either in overdrying the lumber or in drying unnecessarily slowly in an effort to avoid overdrying. To avoid overdrying in many instances, the kiln operator frequently must shut down the kiln long enough to sample the dryness of lumber at several locations within the kiln. These sampling steps, if properly done, can give the operator at least some idea of the additional energy (or drying time) required to reach the desired moisture content throughout the entire batch of lumber in the kiln. This sampling is at best a makeshift and inaccurate expedient, and further increases the overall drying time as the kiln must be shut down one or more times to allow sampling.